The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera.
In these years, for a purpose of improving convenience and security, a plurality of cameras is mounted on a vehicle. For example, a vehicle may be equipped with a back camera to take a picture of a backside of the vehicle. In this case, a driver can see a situation behind the vehicle through a monitor when the vehicle is pulled back, so that even when there is an obstacle that is hard to see due to a blind side of the vehicle, it is possible to safely pull the vehicle back without contacting such an obstacle. There has been an expectation of increase in such a camera mounted on a vehicle, so-called onboard camera.
Usually, an onboard camera is often mounted in a relatively small space such as a backdoor, a front grill, a side mirror, or inside of the vehicle. Therefore, in addition to miniaturization, an imaging lens to be mounted in such an onboard camera is required to attain a higher resolution due to a large number of imaging elements and a wider imaging angle of a view to deal with a wider imaging range. However, it is difficult to attain miniaturization and the high resolution as well as the wider imaging angle of the view, while satisfactorily correcting respective aberrations. For example, if an imaging lens is configured to attain a smaller size, an individual lens has stronger refractive power, so that it is difficult to satisfactorily correct each aberration. For this reason, upon designing an actual imaging lens, it is a key to meet these demands in a balanced manner.
As an imaging lens with the wide imaging angle of the view, for example, Patent Reference has disclosed a conventional imaging lens. The conventional imaging lens is configured to include a first lens that with negative power and a shape of a meniscus lens directing a convex surface thereof to an object side; a second lens that is similarly negative power and is formed to have a shape of a meniscus lens directing a convex surface thereof to the object side; an aperture; a third lens that is biconvex; a fourth lens that has negative power and a shape of a meniscus lens directing a convex surface thereof to the object side; and a fifth lens that is biconvex, arranged in this order from the object side.
With the configuration described above, the imaging lens can attain a small size and satisfactorily correct aberrations by restraining within a certain range a ratio between a distance from a second principal point of a front group composed of the first and second lenses to the aperture and a distance from the second principal point of the front group to a first principal point of a rear group composed of the third through fifth lenses.    Patent Reference: Japanese Patent Publication No. 2003-307674
According to the conventional imaging lens described in Patent Reference, it is possible to attain a wider imaging angle of a view and also relatively satisfactorily correct aberrations although the number of lenses to compose the imaging lens is as few as five. However, since the total length of the lens system becomes long, the imaging lens does not meet the recent demands for miniaturization, and there remains an issue of attaining miniaturization of the imaging lens and satisfactory aberration correction in a good balance. Such an issue is not specific to the imaging lens to be mounted on an onboard camera, and is common among imaging lenses to be mounted on relatively small cameras such as cellular phones, digital still cameras, portable information terminals, security cameras, or network cameras.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that has a wider imaging angle of a view and can satisfactorily correct aberration in spite of a small size thereof.